Accompanying the rapid technology progress of optical information media in recent years, reading speed of optical disk drives which serve as computer peripheral devices also has great improvements.
However, in practical applications the read write speed of optical disk drives is limited by related components such as motor spindle rotation speed, frequency spectrum of decoding chips, optical head characteristics and sledding speed of the optical head frame, etc. To effectively improve the read write speed of the optical disk drives, the performances of the associated components also have to be enhanced.
During operation of the optical disk drives, the sled mechanism of the optical head frame transports the optical head to a preliminary seek position where the data to be read are located, then an objective lens performs a fine search process to read the data on the optical disk. Hence increasing the moving speed of the sled mechanism of the optical head frame to transport the optical head quickly can effectively reduce data reading time of the optical disk drives. Furthermore, besides for broadcasting video images, in most other applications the optical disk drives are operated under random access mode. The design of the sled mechanism directly affects data retrieving speed and reliability of the optical disk drives.
In the designs of optical disk drive for reading data across a large track span, the optical head is driven by a sled mechanism which generally have two types of constructions as follows:
1. driven by a DC motor through rack and pinion; or
2. driven by a stepping motor through lead screw and rack.
Referring to FIG. 1, a conventional sled mechanism 1a comprises a lead screw 10a, a rack 11a and an elastic blade 12. The lead screw 10a is driven by a motor 13a to rotate, and has two ends supported by a support frame 14a. The rack 11a includes a bracket 15a and a resilient section 16a which has a teeth section 17a located thereon. The elastic blade 12 has one end fastened to the bracket 15a through a screw 18a and another end exerting a resilient force on the resilient section 16a to make the teeth section 17a engaging with the screw threads 19a of the lead screw 10a. 
The bracket 15a of the rack 11a is attached to the optical head frame (not shown in the drawing). Through the driving (positive rotation or reverse rotation) of the motor 13a, the optical head (also not shown in the drawing) may be moved reciprocally and axially and horizontally along the lead screw 10a. 
Referring to FIG. 2, in the sled mechanism 1a set forth above, the elastic blade 12a provides the resilient section 16a with sufficient pre-load force to make the teeth section 17a of the rack 11a engaging with the screw threads 19a of the lead screw 10a. 
However, the action force F of the pre-load force incurs two reaction forces A and B at two ends of the sled screw 10a supported by the support frame 14a. Hence the rotation of the sled screw 10a driven by the motor 13a has to overcome the friction force resulting from the reaction forces A and B. This phenomenon will impair the operation smoothness and reliability of the sled mechanism 1a, and adversely affects data reading speed of the optical disk drive. The conventional construction of the sled mechanism previously discussed indicates that there are still rooms for improvement.